This application claims Paris Convention priority of DE 100 41 677.2 filed Aug. 24, 2000 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a superconducting magnet system for generating a magnetic field in the direction of a z axis in a working volume disposed about z=0, with at least one current-carrying magnet coil and at least one additional, superconductingly closed current path, which can react inductively to changes of the magnetic flux through the area enclosed by same, wherein the magnetic fields generated in the z direction in the working volume by these additional current paths during operation due to induced currents do not exceed 0.1 Tesla. The invention also concerns a method for dimensioning these additional current paths.
A device of this type is disclosed e.g. in U.S. Pat. No. 4,974,113-A.
Superconducting magnet arrangements of this type comprising actively shielded magnets are disclosed e.g. in U.S. Pat. No. 5,329,266 or U.S. Pat. No. 4,926,289.
Superconducting magnets are used for different applications, in particular, magnetic resonance methods, wherein the stability of the magnetic field over time is usually important. The most demanding applications are high-resolution nuclear magnetic resonance spectroscopy (NMR spectroscopy). Field fluctuations with time can be caused by the superconducting magnet itself and also by its surroundings. While modern magnet and conductor technology can produce fields which are very constant with time, there is still need for development in the field of suppression of external magnetic disturbances. We will describe means for counteracting these disturbances. The main focus thereby is disturbance compensation with superconducting solenoid magnets having active stray field shielding.
U.S. Pat. No. 4,974,113 describes i.a. a compensating superconducting solenoid magnet, however, without active shielding. At least two independent superconducting current paths are constructed using two coaxial superconducting solenoid coils and calculated such that external magnetic field disturbances occurring inside the arrangement are suppressed to a residual value in long-term behavior of not more than 20% of the original disturbance, thereby taking into consideration conservation of total magnetic flux for each closed superconducting current path. U.S. Pat. No. 4,974,113 further describes a method for calculating the disturbance behavior for such arrangements which is based on the principle of conservation of magnetic flux through a closed superconducting loop.
U.S. Pat. No. 5,329,266 describes an application of this idea to an actively shielded magnet system. A plurality of shielding, structured compensation coils are connected in superconducting series and have a current carrying capacity which is low compared to that of the main coils (on the order of at most one ampere) to ensure that, in case of a superconducting breakdown (=quench), the disturbance field outwardly radiated by the magnet arrangement remains as small as possible.
U.S. Pat. No. 4,926,289 shows an alternative approach which describes an actively shielded superconducting magnet system with a radially inner and a radially outer superconductingly short-circuited coil system, wherein a superconducting short-circuit with limited current carrying capacity is provided between the inner and the outer coil system, such that the current difference between the two coil systems is limited. To compensate for external disturbances, the superconducting current limiter between the two coil systems can produce a shift in the current distribution between the radially inner and the radially outer superconducting current path. In case of a quench, the small current carrying capacity of the current limiter ensures that the external stray field produced by the magnet arrangement remains small.
If additional current paths are dimensioned according to the above-mentioned teaching, the desired compensation effect is difficult to obtain in certain cases. With actively shielded magnets having only one individual superconductingly short-circuited current path, the observed disturbance behavior differs considerably from that calculated according to the above cited prior art. The reason therefor is that, in conventional methods for calculation of the disturbance behavior of a superconducting magnet arrangement, the superconductor is treated as non-magnetic material. The present invention also takes into consideration the fact that the superconductor mainly behaves as a diamagnetic material with respect to field fluctuations of less than 0.1 Tesla and thereby largely expels small field fluctuations from its volume. This results in a redistribution of the magnetic flux of the field fluctuations in the magnet arrangement which then influences the reaction of the superconducting magnet and additional superconductingly closed current paths to an external disturbance, since this reaction is determined by the principle of conservation of the magnetic flux through a closed superconducting loop.
In contrast thereto, it is the object of the present invention to modify a magnet arrangement of the above mentioned type with as easy and simple means as possible such that the disturbance behavior of the magnet system is corrected to an optimum degree by taking into consideration the diamagnetism of the superconductor. The object of the present invention is thereby not limited to modifying a magnet arrangement of the above mentioned type such that external field fluctuations in the working volume of the magnet arrangement are largely suppressed. Arrangements can also be designed which either amplify or weaken external field fluctuations to a certain degree. Such applications are desired e.g. when the external field fluctuation is generated by field modulation coils whose effect in the working volume should be as strong as possible.
This object is achieved in accordance with the invention in that the magnet coil(s) and the additional current path(s) are designed such that, in response to an additional disturbance coil which generates a substantially homogeneous disturbance field in the magnetic volume, the value xcex2 (that factor by which the disturbance is increased or weakened by the reaction of the magnet) is calculated according to   β  =      1    -                  g        T            ·              (                                            (                                                L                  cl                                -                                  α                  ⁢                                      xe2x80x83                                    ⁢                                      L                    cor                                                              )                                      -              1                                ⁢                                    (                                                L                                      ←                    D                                    cl                                -                                  α                  ⁢                                      xe2x80x83                                    ⁢                                      L                                          ←                      D                                        cor                                                              )                                      g              D                                      )            
if and only if this value differs by more than 0.1 from a value       β    0    =      1    -                  g        T            ·              (                                            (                              L                cl                            )                                      -              1                                ⁢                                    L                              ←                D                            cl                                      g              D                                      )            
which would result if xcex1=0.
The above variables have the following definitions:
xe2x88x92xcex1: average magnetic susceptibility in the volume of the magnet coil(s) with respect to field fluctuations which do not exceed a magnitude of 0.1 T, wherein 0 less than xcex1xe2x89xa61,
gT=(gM, gP1, . . . , gPj, . . . gPn),
gPj: field per ampere of the current path Pj in the working volume without the field contributions of the current paths Pi for ixe2x89xa0j and the magnet coil(s),
gM: field per ampere of the magnet coil(s) in the working volume without the field contributions of the current paths,
gD: field per ampere of the disturbing coil in the working volume without the field contributions of the current paths and the magnet coil(s),
Lcl: matrix of the inductive couplings between the magnet coil(s) and the current paths and among the current paths,
Lcor: correction for inductance matrix Lcl, which would result with complete diamagnetic expulsion of disturbance fields from the volume of the magnet coil(s);
L←Dcl: vector of inductive couplings between the disturbance coil and the magnet coil(s) and current paths;
L←Dcor: correction for the coupling vector L←Dcl, which would result with complete diamagnetic expulsion of disturbance fields from the volume of the magnet coil(s).
To improve the disturbance behavior of the magnet, additional current paths are added to the superconducting magnet. These additional current paths must be correctly dimensioned in order to achieve the desired effect. According to the above-cited prior art, this would mean that their field efficiency gPj and the field efficiency gM of the magnet as well as the mutual inductive couplings of the additional current paths among themselves, with the magnet and with the external field sources in addition to self-inductances are correctly calculated and taken into consideration when designing the coils of the current paths. However, when dimensioning the additional current paths in an inventive arrangement, in addition to the above-mentioned coil properties, the magnetic shielding behavior of the superconducting volume portion of the magnet is also taken into consideration.
This shielding behavior appears in all superconducting magnet systems, but only has significant effect on the disturbance behavior in special configurations. Only such special configurations are the object of the invention since, in all other arrangements, the dimensioning of the coil according to the cited prior art already produces satisfying results. The advantage of an inventive arrangement, in which the above-mentioned magnetic shielding behavior of the magnet has significant effect on the disturbance behavior of the arrangement, is that one can assure that the behavior of the arrangement in response to external magnetic disturbances corresponds to expectations. The present invention is thereby not limited to arrangements which largely suppress external field fluctuations in their working volume. On the contrary, it is also possible to design arrangements which amplify or weaken external field fluctuations to a certain extent.
One embodiment of the inventive magnet arrangement is particularly preferred with which the superconducting magnet comprises a radially inner and a radially outer coaxial coil system which are electrically connected in series, wherein these two coil systems each produce one magnetic field in the working volume with opposing direction along the z axis.
In such an arrangement, the magnetic shielding behavior of the superconductor in the magnet usually has a particularly strong effect on the disturbance behavior of the magnet arrangement.
In a further development of this embodiment, the radially inner coil system and the radially outer coil system have dipole moments of approximately equal and opposite strength. This is the condition for optimum suppression of the stray field of the magnet. Due to the large technical importance of actively shielded magnets, the correct dimensioning of additional coils in such magnets, including those cases where the above-mentioned magnetic shielding behavior of the superconductor in the magnet significantly influences the effect of the additional current paths, is very advantageous.
In another advantageous further development of the above-mentioned embodiment, the magnet coil(s) form(s) a first current path which is superconductingly short-circuited during operation and a disturbance compensation coil, which is not galvanically connected to the magnet, is disposed coaxially with respect to the magnet to form a further current path which is superconductingly short-circuited during operation. This embodiment constitutes a simple, realistic solution with only two superconductingly closed current paths. Only one single superconducting current path is provided in addition to the superconducting path of the magnet itself.
In a further advantageous development, at least one of the additional current paths is a portion of the magnet bridged with a superconducting switch. This permits optimization of the disturbance behavior of the magnet arrangement without providing additional coils.
In a particularly preferred embodiment of the inventive magnet arrangement, the current paths which are superconductingly short-circuited during operation are substantially inductively decoupled. In this manner, charging does not produce mutual induction of currents which would be converted into a great amount of heat in the open switches. Moreover, drifting superconducting current paths do not influence one another which could otherwise lead e.g. to a monotonically increasing charging of a coil. During a quench of a superconducting current path, e.g. the magnet, no enhanced stray field is suddenly produced by another current path, such as a compensation coil.
In a particularly advantageous further development of this magnet arrangement, a different polarity of the radially inner coil system and the radially outer coil system is used for inductive decoupling. The utilization of the different polarities of stray field shielding and main coil facilitates the design of magnet arrangements in accordance with the above-described embodiment.
The above-mentioned advantages of the invention are particularly important in sensitive systems. For this reason, in a preferred embodiment, the inventive magnet arrangement is part of an apparatus for high-resolution magnetic resonance spectroscopy, e.g. in the field of NMR, ICR or MRI.
In an advantageous further development of this embodiment, the magnetic resonance apparatus comprises a means for field locking the magnetic field generated in the working volume. Optimization of the disturbance behavior of the magnet arrangement with additional current paths effectively supports the NMR lock.
It should, however, be guaranteed that existing active devices for compensating magnetic field fluctuations, such as the NMR lock, do not interact with the inventive method for eliminating disturbances of the magnet. For this reason, a further development of the above embodiment provides that the inductive couplings between the superconducting current paths and the lock coil are small compared to the corresponding self-inductances of the superconducting current paths. By inductively decoupling the superconducting current paths from the lock coil the effect of the NMR lock is advantageously not impaired by the superconducting current paths.
In another improved further development, the magnet arrangement can also comprise field modulation coils. In such an arrangement, the present invention can guarantee that the superconducting current paths neither obstruct nor amplify the effect of the field modulation coils in the working volume of the magnet arrangement.
In a further advantageous embodiment of the invention, at least one of the additional current paths comprises a superconductingly closed coil which is electrically separated from the magnet arrangement. The use of several additional current paths offers more possibilities to optimize the disturbance behavior of the magnet arrangement.
One embodiment of the inventive magnet arrangement is also of particular advantage wherein the absolute value of   β  =      1    -                  g        T            ·              (                                            (                                                L                  cl                                -                                  α                  ⁢                                      xe2x80x83                                    ⁢                                      L                    cor                                                              )                                      -              1                                ⁢                                    (                                                L                                      ←                    D                                    cl                                -                                  α                  ⁢                                      xe2x80x83                                    ⁢                                      L                                          ←                      D                                        cor                                                              )                                      g              D                                      )            
is smaller than 0.1. Under this condition, external field fluctuations in the working volume of the magnet arrangement are reduced by more than 90 percent. This is desirable for most applications.
The present invention also concerns a method for dimensioning the additional current paths in a magnet arrangement, wherein the portion xcex2 of an external field disturbance which enters the working volume of the magnet system, is calculated taking into consideration the current changes induced in the magnet and the additional current paths according to       β    =          1      -                        g          T                ·                  (                                                    (                                                      L                    cl                                    -                                      α                    ⁢                                          xe2x80x83                                        ⁢                                          L                      cor                                                                      )                                            -                1                                      ⁢                                          (                                                      L                                          ←                      D                                        cl                                    -                                      α                    ⁢                                          xe2x80x83                                        ⁢                                          L                                              ←                        D                                            cor                                                                      )                                            g                D                                              )                      ,
wherein the variables have the above-mentioned definition. This method for dimensioning the additional current paths advantageously takes the magnetic shielding behavior of the superconductor in the magnet into consideration. All embodiments of the invention can be dimensioned with this method through calculation of the behavior of the magnet system when external field disturbances occur thereby taking into consideration the current changes induced in the magnet and in the additional current paths. The method is based on the calculation of correction terms for the mutual inductive couplings among the additional current paths themselves and with the magnet and the external field sources as well as for all self-inductances, these correction terms being weighted with a factor xcex1 and subtracted from their corresponding classically calculated quantities. This method achieves a better correspondence between calculated and measurable disturbance behavior of the magnet arrangement than does the conventional method.
In a simple variant of the inventive method, the parameter xcex1 corresponds to the volume portion of superconductor material in the coil volume of the magnet. This method for determining the parameter xcex1 is based on the assumption that the susceptibility in the superconductor with respect to field fluctuations is (xe2x88x921) (ideal diamagnetism).
The values for xcex1 determined in this fashion cannot be experimentally confirmed for most magnet types. A particularly preferred alternative method variant is therefore characterized in that the parameter xcex1 is experimentally determined for the magnet arrangement from the measurement of the value xcex2exp of the magnet coil(s), with no additional current paths, in response to a disturbance coil producing a substantially homogeneous disturbance field in the magnet volume, with insertion of the value xcex2exp into the equation       α    =                                                      g              D                        ⁡                          (                              L                M                cl                            )                                2                ⁢                  (                                    β                              e                ⁢                                  xe2x80x83                                ⁢                x                ⁢                                  xe2x80x83                                ⁢                p                                      -                          β              cl                                )                                                                g              D                        ⁡                          (                                                β                                      e                    ⁢                                          xe2x80x83                                        ⁢                    x                    ⁢                                          xe2x80x83                                        ⁢                    p                                                  -                                  β                  cl                                            )                                ⁢                      L            M            cl                    ⁢                      L            M            cor                          -                              g            M                    ⁡                      (                                                            L                                      M                    ←                    D                                    cl                                ⁢                                  L                  M                  cor                                            -                                                L                                      M                    ←                    D                                    cor                                ⁢                                  L                  M                  cl                                                      )                                ,
wherein             β      cl        =          1      -                        g          M                ·                  (                                    L                              M                ←                D                            cl                                                      L                M                cl                            ·                              g                D                                              )                      ,
gM: field per ampere of the magnet coil(s) in the working volume,
gD: field per ampere of the disturbance coil in the working volume without the field contribution of the magnet coil(s),
LMcl: inductance of the magnet coil(s)
LM←Dcl: inductive coupling of the disturbance coil with the magnet coil(s),
LMcor: correction for the magnet inductance LMcl, which would result with complete diamagnetic expulsion of disturbance fields from the volume of the magnet coil(s),
LM←Dcor: correction for inductive coupling LM←Dcl between the disturbance coil and the magnet coil(s) which would result for complete diamagnetic expulsion of disturbing fields from the volume of the magnet coil(s),             β              e        ⁢                  xe2x80x83                ⁢        x        ⁢                  xe2x80x83                ⁢        p              =                  g        D        eff                    g        D              ,
gDeff: measured field change in the working volume of the magnet arrangement per ampere of current in the disturbance coil.
Finally, in a further particularly preferred variant of the inventive method, the corrections Lcor, L←Dcor, LMcor and LM←Dcor are calculated as follows:             L      cor        =          (                                                  L              M              cor                                                          L                              M                ←                Pl                            cor                                            ⋯                                              L                              M                ←                Pn                            cor                                                                          L                              Pl                ←                M                            cor                                                          L              Pl              cor                                            ⋯                                              L                              Pl                ←                Pn                            cor                                                            ⋮                                ⋮                                ⋰                                ⋮                                                              L                              Pn                ←                M                            cor                                                          L                              Pn                ←                Pl                            cor                                            ⋯                                              L              Pn              cor                                          )                  L              ←        D            cor        =          (                                                  L                              M                ←                D                            cor                                                                          L                              Pl                ←                D                            cor                                                            ⋮                                                              L                              Pn                ←                D                            cor                                          )      xe2x80x83LPj←Pkcor=fPj(L(Pj,red,Ra1)←Pkclxe2x88x92L(Pj,red,Ri1)←Pkcl)
xe2x80x83LPj←Dcor=fPj(L(Pj,red,Ra1)←Dclxe2x88x92L(Pj,red,Ri1)←Dcl)
LPj←Mcor=fPj(L(Pj,red,Ra1)←Mclxe2x88x92L(Pj,red,Ri1)←Mcl)             L              M        ←        Pj            cor        =                  L                  1          ←          Pj                cl            -              L                              (                          1              ,              red              ,                              Ri                1                                      )                    ←          Pj                cl            +                                    Ra            1                                R            2                          ⁢                  (                                    L                                                (                                      2                    ,                    red                    ,                                          Ra                      1                                                        )                                ←                Pj                            cl                        -                          L                                                                    2                    ,                    red                    ,                                          Ri                      1                                                        )                                ←                Pj                            cl                                )                                L              M        ←        D            cor        =                  L                  1          ←          D                cl            -              L                              (                          1              ,              red              ,              Ri1                        )                    ←          D                cl            +                                    Ra            1                                R            2                          ⁢                  (                                    L                                                (                                      2                    ,                    red                    ,                                          Ra                      1                                                        )                                ←                D                            cl                        -                          L                                                                    2                    ,                    red                    ,                                          Ri                      1                                                        )                                ←                D                            cl                                )                    xe2x80x83LMcor=L1←1clxe2x88x92L(1,red,Ri1)←1cl+L1←2clxe2x88x92L(1,red,Ri1)←2cl
      L    M    cor    =            L              l        ←        1            cl        -          L                        (                      1            ,            red            ,            Ri1                    )                ←        1            cl        +          L              1        ←        2            cl        -          L                        (                      1            ,            red            ,            Ri1                    )                ←        2            cl        +                            Ra          1                          R          2                    ⁢              (                              L                                          (                                  2                  ,                  red                  ,                                      Ra                    1                                                  )                            ←              2                        cl                    -                      L                                          (                                  2                  ,                  red                  ,                                      Ri                    1                                                  )                            ←              2                        cl                    +                      L                                          (                                  2                  ,                  red                  ,                                      Ra                    1                                                  )                            ←              1                        cl                    -                      L                                          (                                  2                  ,                  red                  ,                                      Ri                    1                                                  )                            ←              1                        cl                          )            
wherein
Ra1: outer radius of the magnet coil(s) (in case of an actively shielded magnet arrangement, the outer radius of the main coil),
Ri1: inside radius of the magnet coil(s),
R2: in case of an actively shielded magnet arrangement the medium radius of shielding, otherwise infinite,
RPj: medium radius of the additional coil Pj,       f    Pj    =      {                                                                                        Ra                  1                                                  R                  Pj                                            ,                                                R                  Pj                                 greater than                                   Ra                  1                                                                                                                        1                ,                                                      R                    Pj                                     less than                                       Ra                    1                                                              ⁢                              xe2x80x83                                                        ,      
wherein the index 1 designates the main coil for an actively shielded magnet arrangement, and otherwise designates the magnet coil(s), and the index 2 designates the shielding of an actively shielded magnet arrangement, wherein terms with index 2 are otherwise omitted and the index (X, red, R) designates a hypothetical coil X whose entire windings are wound at the radius R.
The particular advantage of this method for calculating the corrections Lcor, L←Dcor, LMcor and LM←Dcor consists in that the corrections are based on the inductive couplings and the self-inductance of coils, taking into consideration their geometrical arrangement.
Further advantages of the invention can be extracted from the description and the drawing. The features mentioned above and below can be used in accordance with the invention either individually or collectively in any arbitrary combination. The embodiments shown and described are not to be understood as exhaustive enumeration but rather have exemplary character for describing the invention.
The invention is shown in the drawing and explained in more detail with respect to embodiments.